Catalytic reforming, using Pt based reforming catalyst, is one of the most important refinery processes in the world. Most refineries have a catalytic reformer, which converts naphtha fractions into high octane reformate.
Reformers come in many types and sizes--from 2000 BPD fixed bed units to moving or swing bed units processing more than 50,000 BPD. Reformers are available with fixed bed reactors, swing bed reactors, or moving bed reactors. Many new units are moving bed reactors, available from UOP, Inc, Des Plaines, Ill.
Reformers generally use mono-metallic catalysts (Pt on a support such as alumina) or bi-metallic catalyst (Pt-Re on a support). Other combinations of Pt and other metals are known.
All reforming catalysts are believed to contain halogen, almost invariably chlorine. Chlorine is now ubiquitous in catalytic reforming. Chloroplatinic acid may be used in the impregnation solution forming the catalyst. Some refiners may add chlorine compounds during normal operation.
One major oil company developed a Pt reforming catalyst regeneration or "rejuvenation" procedure which conducted at least some portions of the regeneration in the presence of one or more chlorine compounds. The procedure was believed originally developed for swing reactor systems which were regenerated every day or so, but this regeneration method, or some variant of it, was eventually used in semi-regenerative reformers and in moving bed reformers.
All of this chlorine can, and does, find its way into gas and liquid products from the reformer. Based on a review of several decades of The Oil and Gas Journal, the key to successful catalytic reforming is lots of chloride. For decades refiners have talked about the problems of getting enough chlorides into the system, and dealing with the chlorides in the vapor and liquid products from the reformer.
In 1977 there was talk of the need for heat, chloride and moisture to redistribute platinum.
In 1980 there was a discussion of deposits of ammonium chloride in catalytic reforming compressor internals.
In 1985 there was discussion of the need for, and difficulty of maintaining, 1.0 wt % chloride on bimetallic catalyst between regenerations. It was suggested to "come out on the high side on chloride. "
In Alumina adsorbents effectively remove HCl from reformer H2 gas stream, Janke et al, Oil and Gas Journal, May 12, 1986, page 64, talked about controlled injection of organic chloride at the reformer reactor inlet, and the mischief caused by all this chloride. The problem was worse with continuous catalytic reforming processes, which were reported "to require higher levels of chloride addition for regeneration . . ." The solution proposed by the authors was use of alumina adsorbents to remove the HCl from the net off gas. This article is incorporated by reference.
In Apr. 1, 1994 there was a discussion of the problem of corrosion in fired heaters due to chloride in the hydrogen from the reforming unit. The proposed solution was to install alumina treaters.
The problem is not limited to reformers. Similar problems occur in some isomerization units, and may occur in other units which are relatively dry and use a chloride containing catalyst.
The conditions which lead to chloride problems are catalysts which contain, or reaction conditions which require, chlorine compounds, and reactants which are dry enough that no separate aqueous phase forms in the vapor/liquid separator downstream of the reactor. Essentially all Pt reformers meet these conditions, and many isomerization and other processing units meet these conditions.
The situation could be summarized as follows for Pt reformers. Although refiners may use different reforming catalysts, all the catalysts seem to contain chlorine. There is enough chlorine either present in the virgin catalyst, or from chlorine addition during reformer operation, or from chlorine added during the catalyst regeneration, so that chlorine compounds appear in all the product streams coming from the reformer. Both vapor and liquid products have chlorine compounds.
The raw liquid reformate has chlorides. The net hydrogen gas make has chlorine compounds. When the raw reformate is fractionated, usually in a debutanizer, the overhead fractions contain chlorine compounds.
While chlorides in liquid reformate are a serious problem, the present invention is not directed to solving that problem. Instead, the present invention focuses on removal of chlorides or other acidic halides present in gas streams from the reformer. Of primary concern is removal of chlorides from the net gas make from the vapor liquid separator. Another concern is removal of chlorides from vapor streams generated by downstream processing of raw reformate, e.g., removing chlorides from overhead separator vapor associated with reformate fractionators.
Thus the process of the present invention focusses on removal of chlorides from gas streams, rather than from liquid streams.
It should also be clarified that while most reformers use chlorines as a catalyst component, some may use other halogens, such as F or I, but C1 is the halogen of choice, so hereafter chlorine and its reaction or degradation products will be referred to rather than halogens in general.
To solve the problem of removing chlorides from gas streams, refiners have generally used beds of solid adsorbents, such as alumina impregnated with an alkaline material such as NaOH. Such approaches are discussed in the 1994 and 1986 OGJ articles discussed above. While these approaches work, there are problems associated such alumina beds. The problems can include one or more of: cost, catalytic activity, regeneration and disposal.
Alumina beds are relatively costly, in terms of the amount of active ingredient present. The alumina material typically contains 5 to 10 wt % caustic. Alumina costs much more than caustic, and the alumina primarily serves as a support, but one which unfortunately is not always inert.
Alumina beds can exhibit catalytic activity. When alumina beds are used to remove chlorides from flowing vapor streams, aluminum chloride can form, and cause catalytic reactions which convert some of the hydrocarbon vapor species into a much higher molecular weight material. In some units, the gas is turned to goo, at least enough is formed that the effectiveness of the alumina bed is much impaired. This heavy viscous material must be removed to "regenerate" the alumina bed, so that it may be used to absorb additional amounts of chlorides or other acidic components from the flowing gas stream. Steam stripping will usually "regenerate" such a bed.
Disposal of solid adsorbents can be a serious waste management problem. Solid bed adsorbents must eventually be retired and the bed frequently contains too much hydrocarbon to permit the material to be dumped into a landfill. The adsorbent bed may be steam stripped as a prelude to disposal. The resulting water/hydrocarbon product must be stripped to remove benzene from the waste water.
I studied the problem of chloride removal from reformer vapor streams and realized that much of the problem could be overcome by a different approach, which ignored much of the conventional wisdom in gas treating.
While I still made use of a simple acid/base neutralization reaction, my approach used concentrated solid caustic, rather than caustic on some form of support. Rather than use finely divided solid caustic--which one would intuitively think would be better for vapor/solid contact--I used a low surface area material, preferably of large size and with little porosity.
I found a way to permit the process to run almost continuously when treating relatively cool, dry gases such as reformer recycle gas, by coating the bed with liquid hydrocarbon. This should have been a barrier or impediment to the neutralization reaction, but instead allowed the bed to operate for extended periods without plugging, while continuously producing a brine product. Salt was produced continuously, and continuously removed as brine.
This new approach to gas treating allowed significant modifications to some refinery processes. In treating reformer recycle gas I was able to remove a significant amount of the chloride present in the recycle gas. Much of the chloride that the reformer feed, or the reformate, "sees" is simply the chloride present in the recycle gas.
The recycle gas outnumbers the feed, on a molar basis. Removing much of the fugitive chloride from the recycle gas could reduce the chloride loading of the reformate. There would still be some chloride in reformate, due to extraction of chlorides from the reforming catalyst, but the problem would be reduced. This could be used to reduce the amount of chloride in the liquid reformate stream, as well as reduce chlorides in the net gas make of the reformer.
The process could also be used to treat only the net gas make, or excess recycle gas make which is removed as one of the vapor phase products of the platinum reformer. While this stream usually is not considered corrosive (it typically has less than 10 ppm water and only a few ppm chlorides) the catalytic uses to which this hydrogen rich stream is sometimes put can make the chloride content a significant problem.
The debutanizer overhead gas make from a reformer may also be treated. Although a relatively small vapor stream, it typically has a much higher chloride content than any other gas stream associated with the reformer, and usually must be treated for chloride removal before use as fuel or in other refinery processes.
Now acidic gas streams can be treated without formation of viscous oils, as occurred with some adsorbents. Adsorbent swelling and bed plugging are eliminated. High capacity and reactivity of my new bed permit use of smaller reactors and longer cycle lengths.
All of these things are made possible by using a bed of solid caustic or other concentrated alkaline material wetted with or immersed in liquid hydrocarbons to remove acidic halides from gas streams.
This simple change, wetting a solid bed with liquid hydrocarbon, should have made the process worse. It would, at a minimum, add an insoluble hydrocarbon layer which might be thought to isolate the solid caustic from the gas stream.
In practice, the process can be even more complex and may involve the following:
a. a solid caustic material PA1 b. an aqueous film on the caustic PA1 c. a hydrocarbon phase covering the aqueous film PA1 d. a gas phase sweeping by the aqueous phase.
A process involving a solid phase, two immiscible liquid phases, and a gas phase would not normally be considered for an efficient remover of acidic components from a flowing vapor stream, but my experiments show that it works well.
It is almost like some biological processes. It is both complex and efficient. There are four phases involved, one solid, two immiscible liquid phases, and a gas phase.
As the process works simultaneously using four phases, it might properly be termed a quadriphase extraction process, and such terminology is used extensively in this specification as an efficient but brief way to describe this complex process.
An additional feature of the quadriphase system is that it can remove acidic compounds from a gas stream using, indirectly, an aqueous absorbent, without adding much water to the flowing gas stream. At least much less water is added to the gas than would be the case if an aqueous solution of liquid caustic were used to treat the gas.
The process is unusual in that it permits use of pure caustic as a reagent, but the bed does not plug or fuse even though significant amounts of solid caustic are eventually consumed.
The quadriphase process can run for weeks and months even when processing cool gas containing significant amounts of moisture and chlorides. This process can process for extended periods a gas which rapidly plug a solid bed of caustic run without oil. The quadriphase extraction process ran on such gases for months, while a more conventional bed of solid caustic (not oil coated) fused and effectively plugged after a short period of operation in both up and down gas flow. There was no way I could use solid caustic by itself and have a process which would work for a long time to treat such gasses with significant amounts of chlorides and moisture.
The solid caustic could, if coated in oil, be used in a mechanically simply but chemically complex process, involving multiple immiscible liquids to treat gas streams efficiently to remove acidic halides and even to continuously regenerate the solid bed by continuously removing salts as brine.
The above discussion focused primarily on cool gas streams, such as those gas streams from Pt reformers. Using quadriphase extraction, these gas streams could be successfully treated for extended periods using beds of solid caustics. Such beds would plug within hours or days if attempts were made to simply use such a bed without the hydrocarbon layer on the aqueous layer.
Having established an effective way to remove chlorides from gas using the above multi-phase system, I modified this technology to be able to treat dry gas which did not have enough native water, or enough water of neutralization, to maintain a suitable brine phase. Hydrating the dry gas, or spiking the bed with water, allowed even these dry gas streams to be treated using the quadriphase approach. Monitoring of the pH of the brine produced during quadriphase treating also gave a powerful, reliable, and inexpensive way to optimize caustic consumption, and minimize the amount and alkalinity of the brine by-product.